Question

A lens of focal length 12 cm forms an erect image three times the size of the object. The distance between the object and image is :

(a) 8 cm 

(b) 16 cm 

(c) 24 cm 

(d) 36 cm

Answer 1

(b) 16 cm

Magnification, m = v/u

3 = v/u

v = 3u

Lens formula : 1/v - 1/u = 1/f

1/3u - 1/u = 1/12

-2/3u = 1/12

u = -8 cm

v = 3 x -8 = -24 cm

Distance between object and image = u - v = -8 - (-24) = -8 + 24 = 16 cm